The number of low-income preschoolers who qualify as obese or "extremely obese" has dropped over the last decade, new data from the Centers for Disease Control and Prevention show.
Although the decline was only "modest" and may not apply to all children, researchers said it was still encouraging.
"It's extremely important to make sure we're monitoring obesity in this low-income group," said the CDC's Heidi Blanck, who worked on the study.
Those kids are known to be at higher risk of obesity than their well-off peers, in part because access to healthy food is often limited in poorer neighborhoods.
The new results can't prove what's behind the progress, Blanck told Reuters Health - but two possible contributors are higher rates of breastfeeding and rising awareness of the importance of physical activity even for very young kids.
Blanck and her colleagues used data on routine clinic visits for about half of all U.S. kids eligible for federal nutrition programs - including 27.5 million children between age two and four.
They found 13 percent of those preschoolers were obese in 1998. That grew to just above 15 percent in 2003, but dropped slightly below 15 percent in 2010, the most recent study year included.
Similarly, the prevalence of extreme obesity increased from nearly 1.8 percent in 1998 to 2.2 percent in 2003, then dropped back to just below 2.1 percent in 2010, the research team reported Tuesday in the Journal of the American Medical Association.
Whether kids are obese is determined by their body mass index (BMI) - a measure of weight in relation to height - and by their age and sex.
For example, a four-year-old girl who is 40 inches tall would be obese if she was 42 pounds or heavier. A two-year-old boy who is 35 inches tall qualifies as obese at 34 pounds or above, according to the CDC's child BMI calculator. (The CDC's BMI calculator for children and teens is available here:.)
The new findings are the first national data to show obesity and extreme obesity may be declining in young children, Blanck said.
"This is very encouraging considering the recent effort made in the field including by several U.S. federal agencies to combat the childhood obesity epidemic," said Dr. Youfa Wang, head of the Johns Hopkins Global Center on Childhood Obesity in Baltimore.
Blanck said between 2003 and 2010 researchers also saw an increase in breastfeeding of low-income infants. Breastfeeding has been tied to a healthier weight in early childhood.
Additionally, states and communities have started working with child care centers to make sure kids have time to run around and that healthy foods are on the lunch menu, she added.
Parents can encourage better eating by having fruits and vegetables available at snack time and allowing their young kids to help with meal preparation, Blanck said.
Her other recommendations include making sure preschoolers get at least one hour of activity every day and keeping television sets out of the bedroom.
"The prevalence of overweight and obesity in many countries including in the U.S. is still very high," Wang, who wasn't involved in the new study, told Reuters Health in an email.
"The recent level off should not be taken as a reason to reduce the effort to fight the obesity epidemic."